The invention relates to a method for carboxylating terminal alkynes, as defined in the preamble to the first claim.
A method of this type is known from the publication by S. Dxc3xa9rien, J.-C. Clinet, E. Dunach and J. Pxc3xa9richon: xe2x80x9cActivation of Carbon Dioxide: Nickel-Catalyzed Electrochemical Carboxylation of Diynes,xe2x80x9d J. Org. Chem. 1993, 58, 2578-2588. In this method, a dialkyne that has been dissolved in dimethyl formamide is placed in a pressure-tight electrolysis cell without a membrane, and carboxylated with gaseous carbon dioxide under a pressure of 1 to 5 bar. For the carboxylation process, a catalyst comprising an LNi (0) complex is used, with L representing an organic chemical ligand. The catalyst is created on the spot from the corresponding ligand and Ni (II) salts through electrolysis. A carbon fiber as a cathode, and a magnesium anode, serve as electrodes of the electrolysis cell.
This carboxylation yields a series of different carboxylic acids. On the one hand, aromatic or cycloaliphatic carboxylic acids are formed through cyclization. In addition to carboxylic acids with terminal carboxyl groups, carboxylic acids having secondary carboxyl groups are formed. A feature common to all of the reaction products, however, is that this terminal triple bond of the alkyne involved in the carboxylation is converted into a double bond. Therefore, this method cannot be used to produce 2-alkyne carboxylic acids from the terminal alkynes.
In xe2x80x9cLigand-directed reaction products in the nickel-catalyzed electrochemical carboxylation of terminal alkynes,xe2x80x9d Journal of Organometallic Chemistry, 353 (1988), C51-C56, E. Labbxc3xa9, E. Dunach and J. Pxc3xa9richon describe the influence of N and P ligands for use in nickel catalysts for electrochemical carboxylation. With one of the nickel catalysts (Ni(cyclam)Br2), 1-octyne reacted to 2-nonyne carboxylic acid with a high selectivity.
E. Dunach and J. Pxc3xa9richon report on further attempts at the electrochemical carboxylation of terminal alkynes with the aid of nickel complex catalysts in xe2x80x9cElectrochemical carboxylation of terminal alkynes catalyzed by nickel complexes: unusual regioselectivity,xe2x80x9d Journal of Organometallic Chemistry, 352 (1988), 239-245. In this carboxylation process, the triple bond is converted to a double bond, so no alkyne carboxylic acids can be formed.
It is the object of the invention to propose a method of the type mentioned at the outset, in which carbon dioxide is selectively inserted between the terminal Cxe2x80x94H bond, with the triple bond being retained. A nickel complex catalyst is to be omitted here.
The object is accomplished by the features of the first claim. The further claims disclose preferred embodiments of the method.
According to the invention, terminal alkynes are carboxylated with gaseous carbon dioxide in an aprotic solvent; the carboxylation is performed in an undivided electrolysis cell without the use of a catalyst or catalyst precursor. Suitable terminal alkynes are, notably, the alkynes having 2 to 9 carbon atoms. Substituents of the terminal alkyne, but also ether functions and additional double or triple bonds, typically do not interfere with the reaction, because the terminal, triple-bonded carbon atom is selectively monocarboxylated with the method of the invention. Terminal alkynes having a further acidic proton, particularly those having a hydroxyl group, enter into secondary reactions, however; they are therefore unsuitable as starting materials. Alkynes having a non-terminal triple bond are not carboxylated according to the invention.
The gaseous carbon dioxide is supplied to the electrolysis under an overpressure, preferably at a pressure of 0.2 to 5 bar. Pressures of 0.5 to 1 bar are especially preferred. Pressures above 1 bar, but especially above 5 bar, effect a reduced yield, and should therefore be avoided. The reaction temperature can be room temperature. Higher or lower temperatures are not significantly advantageous.
According to the invention, the carboxylation is performed in an aprotic solvent. Dimethyl formamide is a particularly suitable solvent; acetonitrile and tetrahydrofuran can also be used, however.
The method of the invention is performed in an undivided electrolysis cell having a cathode and an anode, the cell being sufficiently pressure-tight and having a gas supply line for the carbon dioxide. In principle, metals are suitable as a cathode, but not the carbon fibers used in the prior art. Silver cathodes attain especially high reaction yields. Metals that can oxidize easily, such as zinc and aluminum, are suitable as anode material; magnesium is preferred.
The electrolysis is preferably performed under galvanostatic conditions. Typically, maximum voltages of 20 V and current intensities in a range of 50 mA are established.
The invention is described in detail below in conjunction with figures and embodiments.